Great Slave Lake is one of the largest subarctic lakes in the world and the deepest lake in Canada. The duration and strength of thermal stratification is increasing in many subarctic lakes across the northern hemisphere, but little information is available for larger and deeper systems such as Great Slave Lake. This thesis uses paleolimnological techniques to investigate ecological changes occurring in Yellowknife Bay, Great Slave Lake, over approximately the last two centuries based on diatom and Chla analysis of a dated sediment core. The results of this thesis showed a sharp increase in the relative abundance of the smaller planktonic Discostella stelligera from <10% to ~25% diatoms and a decline in heavier Aulacoseira sp. These data suggest that over the last ~two centuries thermal stratification is increasing in Yellowknife Bay with potentially important ecological consequences for this large subarctic lake.